The various organs and tissues of the human body fall prey to a myriad of different afflictions. For example, each year in the United States alone, approximately 180,000 women are diagnosed with breast cancer and 46,000 women die of this disease. In all, 10 to 11 percent of all women can expect to be affected by breast cancer at some time during their lives. The causes of most breast cancers are not yet understood. Screening and early diagnosis are currently the most effective ways to reduce mortality from this disease.
Currently mammography is the most effective means of detecting non-palpable breast cancer. However, mammography cannot determine whether a lesion is benign or cancerous, typically one or more biopsies must be performed per lesion. Unfortunately the biopsy operation itself is a very traumatic and costly operation that often results in some degree of disfigurement. Therefore it is important to improve the specificity of mammography thereby reducing errors, patient trauma, and disfiguration from unnecessary biopsies. It is also important to reduce health care costs by decreasing the number of unnecessary biopsies. For example, to detect 100,000 non-palpable cancers, approximately 500,000 biopsies must be performed at a cost of about $5,000 per biopsy, yielding a total cost of approximately 2.5 billion dollars. Therefore a reduction of 50 percent would save about 1.25 billion dollars per year.
Palpable mass abnormalities of the breast are often difficult to evaluate mammographically. This is especially true for patients with dense or dysplastic breasts (approximately 35 percent of women over 50 and 70 percent of women under 50) or those patients that exhibit signs of a fibrocystic change, for example due to radiation therapy. For example, invasive lobular carcinoma in dense breasts can attain a size of several centimeters and yet still show no mammographically detectable signs. Furthermore, about 50 percent of all preinvasive cancers do not show mammographically significant calcifications, thus decreasing the chances of detecting the malignant tumors.
Lastly, due to the interpretational limitations of mammography, many high risk patients (i.e., patients with a family history of breast cancer, patients with prior histologic evidence of cellular atypia, patients with a prior history of breast cancer who have undergone lumpectomy and radiation therapy) may be forced to rely on random tissue biopsies performed on suspicious areas. Unfortunately this technique typically results in a high nonmalignant-to-malignant biopsy ratio.
A relatively new scientific tool that has allowed scientists and physicians to address problems in physiology and biochemistry in the human body with low risk is emission computed tomography (ECT). ECT systems are mainly used for the detection and imaging of the radiation produced by radiotracers and radiopharmaceuticals. For example, by administering biologically active radiopharmaceuticals into a patient it is possible to image organ functions in real time.
The two major instruments presently used for ECT are Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). These instruments have been used to study a variety of different organs and conditions including cerebral glucose consumption, protein synthesis evaluation, cerebral blood flow and receptor distribution imaging, oxygen utilization, stroke, heart, lung, epilepsy; breast cancer, dementia, oncology, pharmacokinetics, psychiatric disorders, and radio labelled antibody and cardiac studies. Since the SPECT and PET instruments use different types of radiotracers, the metabolic activities imaged are mostly different leading these two instruments to complement rather than compete with each other. The SPECT detectors have proven especially useful for heart and brain imaging.
SPECT dates back from the early 1960s, when the first transverse section tomographs were presented by Kuhl and Edwards (1963) using a rectilinear scanner and analog back-projection methods. With the availability of computer systems and the impetus of computer-assisted tomography using transmitted x-rays, nuclear medicine instruments were modified, and a number of mathematical approaches to tomographic reconstruction were developed in the early 1970s. Rotating Anger cameras and advances in computers opened the way to three-dimensional SPECT systems. Recently interest in SPECT increased as mathematical reconstruction techniques improved. They allowed for attenuation compensation, scattered radiation correction and the availability of new radiopharmaceuticals with higher uptake in the brain or other organs. The major limiting factors for the SPECT systems presently are the sensitivities (≈10 Cts s−1 μCi−1 point and ≈1,000 Cts s−1 cm−1 volume), resolution (7 to 12 mm FWHM), size, and cost.
Present SPECT systems mainly use the rotating Anger camera. Many different variations of the Anger camera and other smaller size rotating single or dual instruments have been designed and used. Most of the commercial instruments use NaI(Tl), CsI(Tl), CsF, BaF2, BGO and other related crystal detectors. The majority of the commercial instruments use the Anger cameras made of NaI(Tl) crystals. All commercial SPECT instruments use collimators for determination of the direction of the incident gamma rays. The main types are parallel and converging collimators. The converging fan or cone beam collimators produce higher sensitivity but increase the complexity of the data analysis. Pinhole and slit collimators are also used. The collimators for high resolution systems eliminate about 99.9 percent of the incident gamma rays. A typical collimator hole has an area of about 1 square millimeter and a length of 1.9 centimeters. Increasing collimator resolution decreases sensitivity and vice versa. Collimators made of high atomic number materials such as lead which also produce considerable amounts of scattered gamma rays on the inside surface of the collimator, thereby increasing the scattered photon background.
Anger cameras are normally rotated on a gantry around the patient for about 20 minutes to acquire sufficient data for a reasonable image. The spatial resolutions are limited to about 7 to 12 millimeters although spatial resolutions are expected to reach 6 millimeters in the near future. The best energy resolution at gamma ray energies is about 10 percent, limiting the ability of Anger cameras to discriminate scattered photon background. Commercially available SPECT systems include ADAC ARC, GE Starcam, Elscint APEX, Trionix Triad, Digital Scintigraphics ASPECT and University of Michigan SPRINT II.
From the foregoing it is apparent that an improved gamma ray imaging system is desired.